no 



MONOGRArnS OF NOKXn AMERICAN liODICNTIA. 



i!i 



the toolli, sunotiiHlcd by a zigzn<,' onanifl-vvall, wliicli semis into the dentine 

 space, iVom caoli sidi;, two iiidoiitatioiis; these indentations on Ihe outer side 

 bcinj? much deeper tiian those from the inner side, and semicircular in out- 

 line, with convexity forward ; these insulations being loops of enamel, i. e., 

 ciuisisUnj^f of tii(! enamel-sheet folded af,'aiust itself; at the point of the begin- 

 niiig to fold, there is, of course, an external nick or reentrance, and so there 

 are two of these on each side of the tooth, the inner being the more open. 

 In the next stage, a little further abrasion grinds out the continuity of these 

 inlying enamel-folds with tiie general enamel-envelope, because the folds are 

 not so deep down in the substance of the tooth at its edges as they are in the 

 interior ; and then we have the condition of crescentic islands of enamel lying 

 in the general dentine area that is surrounded by the general indcted enamel- 

 wall. This occurs at full nuitiirity. The final condition of senile decline is 

 still something dilferent; for, lastly, these enamel islands are entirely rubbed 

 out, and tiie face of the tooth is one continuous area of dentine, a little exca- 

 vated or sunken l>cl(tw the level of the continuous exterior sheet of enamel 

 that irregularly surrounds it. 



These special details, though readily observed, arc difficult to describe 

 clearly, and the description must l)e followed with specimens in hand. The 

 student may inuigine the top of a pigeon-pie, full of humps and hollows, 

 gradually razeed down by a succession of thin, parallel, horizontal slices. Let 

 the crust be the enamel, and the substance of the pie the dentine ; the first 

 slice will shave off the tops of one or more humps, exposing the interior 

 (dentine) in isolated places, these i.slands lying in a net-work of crust (enamel) ; 

 other slices will make a continuous hole through the crust (enamel), exposing 

 a continuous area (<leiitine) bounded by an irregular wall of crust; and 

 so on. 



All this is very different from the straight upright bundles of prisms that 

 compose the teeth of Arvicolu ; after the bumpy tops of which are once filed 

 down smooth, further abrasion, continued never so long, does not essentially 

 modify tiie pattern of the crowns. 



HESPEROMYS (VESPERIMUS) LEUCOPUS. .; > 



Wbitc-routed or Deer MouM. ' " ^'' -^ 



Amrleaii MM iloiine, or Hat, I'knnant, SjnopBis, 1771, No. 303; Hist. Quod. 1781, No. 30S; Arot. Zool. i, 



1781, 131. 

 American IVandcriiig Home, Bauton, M«d. & Surg. Journ. Pliila. i, 1805, 31 (iioticos a groat migrutiou 



b; Lnke Kric). 



